Muwatta Malik, Book 29, Hadith 1230 (In-Book)

Yahya related to me from Malik from Yahya ibn Said and from Yazid ibn Abdullah ibn Qusayt al-Laythi that Said ibn al-Musayyab said, ''Umar ibn al-Khattab said, 'If a woman is divorced and has one or two periods and then stops menstruating, she must wait nine months. If it is clear that she is pregnant, that is that. If not, she must do an idda of three months after the nine, and then she is free to marry.' "

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، وَعَنْ يَزِيدَ بْنِ عَبْدِ اللَّهِ بْنِ قُسَيْطٍ اللَّيْثِيِّ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، أَنَّهُ قَالَ قَالَ عُمَرُ بْنُ الْخَطَّابِ أَيُّمَا امْرَأَةٍ طُلِّقَتْ فَحَاضَتْ حَيْضَةً أَوْ حَيْضَتَيْنِ ثُمَّ رَفَعَتْهَا حَيْضَتُهَا فَإِنَّهَا تَنْتَظِرُ تِسْعَةَ أَشْهُرٍ فَإِنْ بَانَ بِهَا حَمْلٌ فَذَلِكَ وَإِلاَّ اعْتَدَّتْ بَعْدَ التِّسْعَةِ الأَشْهُرِ ثَلاَثَةَ أَشْهُرٍ ثُمَّ حَلَّتْ ‏.‏

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also referenced under: Book 29, Hadith 70 (USC-MSA)